Field of the Invention
The present invention relates to a method for manufacturing a silicon carbide semiconductor device in which an epitaxial growth layer is formed after gas etching of surface of a silicon carbide single crystal substrate.
Background Art
In recent years, silicon carbide (SiC), which has relatively larger band gap, breakdown field strength, saturated drift velocity, and heat conductivity as compared with silicon, is receiving attention as the power device material for power control. In fact, a power device using this silicon carbide (silicon carbide semiconductor device) can significantly reduce power loss and contribute to downsizing, whereby realizing an energy save during change of a power source; and as a result, this becomes a key device for a low carbon society in which an electric vehicle with higher performance, a more highly functionalized solar system, and the like can be realized.
In production of a silicon carbide semiconductor device, generally, a silicon carbide layer which will become an active region of the silicon carbide semiconductor device is heated in advance for epitaxial growth by means of a thermal CVD method (thermochemical gas phase deposition method) or the like on a silicon carbide single crystal substrate (SiC bulk single crystal substrate). Here, the active region means a cross section region which includes the axis of the growth direction wherein the layer thickness and the doping density in the crystal are precisely controlled. The reason to require not only the silicon carbide single crystal substrate but also the silicon carbide layer like this is because the doping density and the layer thickness are fixed in accordance with the device specification, so that a higher accuracy than that of the bulk singe crystal substrate is wanted.
Hereinafter, the wafer having the silicon carbide layer epitaxially grown on the silicon carbide single crystal substrate is called as an epitaxial wafer. The silicon carbide semiconductor device is manufactured by processing this epitaxial wafer with various treatments. Accordingly, the device yield which is the ratio of the number of the obtained device having an intended properties from one epitaxial wafer is highly dependent on uniformity of electric properties of the epitaxial growth layer. If there is a local region in which the breakdown field strength in the epitaxial wafer surface is smaller than other region, or if there is a local region in which relatively large electricity flows upon application of a certain voltage, device characteristics including the said region, such as for example, the voltage resistance characteristics may be deteriorated. Therefore, even when a relatively smaller voltage is applied, there occurs a problem of the flow of a so-called leak electricity. In other words, the factor to primarily determine the device yield resides in crystallographic uniformity of the epitaxial wafer. As the factor to impair this uniformity, it is known that there are various so-called current leakage defects caused by a problem during the epitaxial growth.
The common feature among the above-mentioned crystal defects resides in that periodicity of atomic array in the crystal is locally imperfect in the direction of the crystal growth. As to the defect caused by the epitaxial growth of the silicon carbide, the current leakage defect that is called as a carrot defect, a triangle defect, and so forth from the feature of its surface form is known. Among them the factor from which the triangle defect is caused includes, besides a different kind of a polytype crystal nucleus and a scar which is caused by polishing and is remained on the substrate surface, a SiC dust which is attached on the substrate surface. This SiC dust is attached during the time when the silicon carbide single crystal substrate is produced, namely this includes the dust that is attached during processing the silicon carbide single crystal substrate by means of carving, polishing, and so forth after growth of the SiC bulk single crystal and the dust that is released from the silicon carbide layer which is deposited inside the reaction furnace during the epitaxial growth on the silicon carbide single crystal substrate by means of the thermal CVD method.
Meanwhile, a technology to flatten the substrate surface by means of hydrogen etching of the silicon carbide single crystal substrate has been proposed (for example, see Japanese Patent Laid-Open Publication No. 2001-77030). However, this did not suppose the case to flatten the silicon carbide single crystal substrate having the flatness with the average roughness of 0.2 nm or less.